Pretreatment of tumor cells with chemotherapeutic agents, metabolic inhibitors, enzymes or hormones modifies the susceptibility of the cells to killing by antibody and complement. The purpose of this investigation is to determine the attributes of the cells which are modified following treatment with the agents. BIBLIOGRAPHIC REFERENCES: Schlager, S.I., Boyle, M.D.P., Ohanian, S.H. and Borsos, T.: Effect of inhibiting DNA, RNA, and protein synthesis of tumor cells on their susceptibility to killing by antibody and complement. Cancer Res. 37: 1432-1437, 1977. Schlager, S.I., Ohanian, S.H. and Borsos, T.: Kinetics of hormone-induced tumor cell resistance to killing by antibody and complement. Cancer Res. 37: 765-770, 1977.